The present invention relates to MEMS devices. More specifically, the present invention relates to MEMS devices having reduced mechanical or thermal bias.
Microelectromechanical systems (MEMS) include very small mechanical devices that are often lithographically etched upon a substrate. These mechanical devices typically include one or more capacitive and/or resistive elements. As the mechanical devices move, for example, the capacitances change, and the change in capacitances are manifested into one or more electrical signals.
The inventors have demonstrated that as fabrication technologies and capabilities increase, the sensitivity of such MEMS devices, e.g. accelerometers, gyroscopes, and the like, of such devices increases. One problem, however, is the effect of stiction forces upon smaller and smaller physical components. To address such forces, mechanical stops and limits are often integrated into the MEMS designs.
Another problem is the effect of physical stresses upon such MEMS devices or substrates of the MEMS devices. Examples of physical forces include expansion/contraction stress of the MEMS substrate or components of the MEMS device due to: temperature stresses of the MEMS device during operation, sometimes due to differences in coefficient of thermal expansion between the substrate and packaging material; mechanical stress from manufacturing machines (e.g. pick and place, wave soldering, socketing) of the MEMS device onto a circuit board; mechanical stress applied of the MEMS device after integration into a hand-held device; or the like. In some cases, such as temperature, these forces are dynamic, and can vary in time.
The inventors are aware of one set of solutions that are used to attempt to compensate for stresses applied to the MEMS device due to changes in temperature. In one solution, each MEMS device is manufactured with a temperature sensor. After packaging, each MEMS device is individually tested in physically controlled environments at a variety of different temperatures. The MEMS device output readings are monitored and then correlated to the temperatures sensed by the internal temperature sensor. Next, a series of calibration numbers for the output readings and associated operating temperature are determined and then stored on the specific MEMS device.
The inventors of the present invention can see numerous drawbacks to the approach above. One drawback includes that the designer of the MEMS device must disadvantageously reserve a certain amount of die area for temperature sensors and the like. Another drawback is that from a manufacturing standpoint, it is very time consuming and expensive to have to individually thermally cycle test each MEMS device and to individually program each MEMS device based upon the thermal cycle test data.
Yet another drawback is that these temperature calibration processes cannot compensate for other stresses that may be imparted to the MEMS device, such as: physical stresses due to integration or insertion of the MEMS device onto a PCB, socket, or the like; physical stresses applied to a device having the MEMS device (e.g. sitting upon a device, or the like); long term drift or biases of such MEMS devices (e.g. hysteresis); and the like. These and other types of stresses, that may or may not be dependent upon time and may affect the outputs of the MEMS devices, are not presently accounted for.
In light of the above, it is desired to have MEMS devices with reduced stress sensitivity without the drawbacks described above.